Preserving Linearity of a RF Power Amplifier

ABSTRACT

A method and circuit for preserving linearity of a RF power amplifier, the power amplifier including a RF power output unit having a characteristic drive level and fed by a supply voltage, comprising measuring the output voltage of the RF power output unit; comparing the measured output voltage to at least one threshold voltage to produce a control signal; and adapting the drive level or the supply voltage of the RF power output unit by means of the control signal to operate the output unit below its saturation level. A method and circuit for stabilizing an antenna circuit comprising a RF power amplifier and a matching circuit by preserving linearity of a RF power amplifier, where the above power amplifier is used.

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 10/527,775, which has a U.S. filing date ofMar. 14, 2005, which is a 35 U.S.C. § 371 national stage entry ofInternational Application No. PCT/IB2003/003422 filed on Aug. 4, 2003,which claims priority benefit under 35 U.S.C. § 119 of European PatentApplication No. 02078854.3 filed on Sep. 17, 2002, to which priority isalso claimed here.

The invention relates to methods and circuits for preserving thelinearity of a RF power amplifier as used for example in RF antennacircuits.

RF antennas as for instance applied in mobile phones, operate instrongly varying environments, resulting in a varying antenna inputimpedance, a VSWR (Voltage Standing Wave Ratio) of 4:1 is not uncommon.Especially at high output levels this may result in a severe distortionof for instance a CDMA, TDMA, Edge or W-CDMA modulated carrier signalhaving a non-constant envelope. The main cause of distortion iscollector voltage saturation of the RF output transistor that occurswhen the collector load impedance is relatively high. The conventionalsolution to protect the power amplifier of a cellular phone againstantenna mismatch conditions to preserve linearity is to use acirculator. The circulator secures proper 50 Ohm loading of the poweramplifier under antenna mismatch conditions by dissipating the reflectedpower in the isolator or in the third circulator port termination.Directivity in the power flow is created by ferromagnetic material.

Another known approach is to apply a fully adaptive system based on thepolar or cartesian loop. Such adaptive systems are described inliterature. Disadvantages of this approach are the resulting rather highcomplexity and the required accurate high speed signal processing,operating at RF frequencies, which leads to a high power consumption.

The above aspects of the state of the art are described in more detailwith reference to FIG. 1 which shows a basic block diagram used for apower source isolated with a circulator from a mismatched antenna. Thecurrent source and its impedance Zo represents an ideal power source(RF-transistor).

Thanks to a circulator the reflected power from the antenna is notreflected towards the source, but dissipated into the circulator load.Consequently P_(refl) _(—) _(circ) and P_(refl) _(—) _(source) as wellas V_(source) and I_(source) are zero. This avoids extremes that wouldoccur when incident and reflected waves could add up in-phase. However,if it is assumed that P_(rad) has to be maintained constant (undercontrol of field strength indication at the base station) the incidentpower has to be increased to overcome reflection losses resulting inenhanced signal voltage and current at the source. Thus, a circulatoronly partly preserves power amplifier linearity under antenna mismatchconditions.

Table 1 below shows typical figures for an impedance matching networkinsertion loss II_(match) of 0.7 dB, a circulator insertion lossII_(circ) of 0.3 dB, a radiated output power P_(rad) of 28.5 dBm and anominal source load impedance Zo of 2 Ohm. The figures relate to a poweramplifier output with circulator for Pout=28 dBm and Zo=2 Ohm.

TABLE 1 Prad/ v or i_source_max/ Pinc_source v⁺ or i⁺_source Pinc_sourcev_source_max i_source_max VSWR_(ant) Γant Γmatch Lin. [dB] Lin. [dB] [W][dBm] [Vrms] [Arms] 1 0 0 0.80 −1.0 1 0 0.891 29.5 1.33 0.667 2 0.33 00.71 −1.5 1 0 0.993 30.0 1.41 0.705 3 0.5 0 0.60 −2.2 1 0 1.180 30.71.53 0.768 4 0.6 0 0.51 −2.9 1 0 1.380 31.4 1.66 0.831 6 0.71 0 0.40−4.0 1 0 1.785 32.5 1.89 0.945 ∞ 1 0 0 −∞ 1 0 ∞ ∞ ∞ ∞

As the figures in Table 1 show, mismatch at the antenna does result inincreased incident signal voltage and current at the source. Forinstance, for an VSWR of I the average source voltage equals 1.33 V rmsor 1.89 Vpk. In edge the modulation peak-to-average ratio is about 3.2dB which results in an RF peak voltage of 2.73V which just about fitswithin a 3V supply and 0.3V collector saturation voltage. For an VSWR of6 the RF peak voltage becomes 1.89*1.414*1.445=3.86V which does not fitwithin a 3V supply.

In case of a power amplifier without a circulator, see FIG. 2, antennaimpedance variations result in load variations of the source. Themaximum source voltage as well as source current is enhancedsignificantly when the incident and reflected wave are in phase.

The Table 2 below shows typical figures for a II_(match) of 0.7 dB andP_(rad) of 28.5 dBm and Zo=2 Ohm

TABLE 2 Prad/ v or i_source_max/ Pinc_source v⁺ or i⁺_source Pinc_sourcev_source_max i_source_max VSWR_(ant) Γant Γmatch Lin. [dB] Lin. [dB] [W][dBm] [Vrms] [Arms] 1 0 0 0.85 −0.7 1 0 0.832 29.2 1.29 0.64 2 0.33 0.280.76 −1.2 1.28 2.1 0.933 29.7 1.75 0.87 3 0.5 0.42 0.64 −1.9 1.42 3.01.096 30.4 2.10 1.05 4 0.6 0.51 0.54 −2.6 1.51 3.6 1.288 31.1 2.42 1.216 0.71 0.61 0.42 −3.7 1.61 4.1 1.659 32.2 2.93 1.47 ∞ 1 0.85 0 −∞ 1.855.3 ∞ ∞ ∞ ∞

For instance, at a VSWR of 4:1 the maximum voltage that can occur at thesource is a factor 1.51 (3.6 dB) larger than under 50 Ohm conditions. Inthis case approx. 31.1−29.2=1.9 dB signal enhancement is due tocompensation of the reflection losses and approx. 1.7 dB due to thereflected signal v⁻ _(source).

The average source voltage, in this case, equals 2.42 Vrms whereas thecollector voltage is only 1.66 Vrms when a circulator is used (see Table1). Therefore, at an VSWR of 4:1 a power amplifier without circulatorcan deliver approximately 20*log(2.42/1.66)=3.3 dB less power forsimilar maximum signal voltage and current at the source than a poweramplifier with circulator.

The problems of preserving the linearity of a RF power amplifier as usedfor example in RF antenna circuits are also addressed in literature, andsome samples are given below.

U.S. Pat. No. 6,064,266 discloses a load limiting circuit and method forlimiting the output impedance seen by an amplifier, wherein a loadlimiting impedance, preferably a resistor, is selectively coupled inparallel with the output impedance by a switch circuit, when a thresholddetect circuit detects the value of the output impedance rising above apredetermined value. The output impedance is preferably measured bymonitoring the output current through a resistor connected in serieswith the output impedance.

U.S. Pat. No. 5,442,322 discloses a control circuit for a multi-stagepower amplifier (such as in a portable radio transmitter) compensatesfor fluctuations in ambient temperature, load, signal level and powersupply voltage. The control voltage is set by comparing a biasing levelwhich is related to the amplifier input signal level to a voltageproportional to the power supply current of the last stage of theamplifier. The control voltage resulting from the comparison establishesthe operating point of the last stage of the power amplifier.

U.S. Pat. No. 4,312,032 discloses an apparatus for providing acontrolled dynamically programmable RF power level to the real part of aterminating load which may vary in impedance over a wide range.Immitance monitoring apparatus produces vector signals representinginstantaneous measurements of the voltage and current of an RF signalsupplied to a load by an RF generator. These signals which containinformation on the phase relationship between the voltage and currentare appropriately processed and then multiplied to produce a DC signalwhich represents the power of the RF signal. The DC signal is employedto control the gain of a variable gain RF amplifier connected betweenthe RF generator and the immittance monitoring apparatus. A feedbackcontrol loop is thus provided which controls the power of the RF signalsupplied to the load. By amplifying, or attenuating, the signals presentat some point in the control loop, the amount of power delivered to theload may be selectively varied.

It is an object of the invention to provide methods and circuits forpreserving the linearity of a RF power amplifier as used for example inRF antenna circuits whereby the power amplifier can perform, underpredetermined load variation, substantially as good as with knownisolation by circulators or separate isolators.

The above object is achieved by a method for preserving linearity of aRF power amplifier, the power amplifier including a RF power output unithaving a characteristic drive level and fed by a supply voltage,comprising measuring the output voltage of the RF power output unit;comparing the measured output voltage to at least one threshold voltageto produce a control signal; and adapting the drive level or the supplyvoltage of the RF power output unit by means of the control signal tooperate the output unit below its saturation level.

It is possible to keep the RF power output unit, in particular thecollector of the RF output transistor, out of saturation by detectingthe output voltage, in particular the peak minimum collector voltage, ofthe RF power output unit and, if necessary, by adapting the poweramplifier drive level or the power amplifier supply voltage. This willpreserve the linearity of the power amplifier.

In a preferred embodiment of the above method where power amplifierincludes a variable gain preamplifier supplying the drive voltage to theRF power output unit, the control signal is used to adapt the gain ofthe preamplifier. The variable gain preamplifier is used to keep the RFpower output unit out of saturation and thus preserving the linearity ofthe RF power output unit.

In a preferred embodiment of the above method the control signal iscombined with the gain control signal of the preamplifier which is anadvantageous way of feeding the control signal to then pre-amplifier.

The above object is achieved by a method for controlling an antennacircuit comprising a RF power amplifier and a matching circuit bypreserving linearity of a RF power amplifier, the power amplifiercomprising a RF power output unit having a characteristic drive leveland fed by a supply voltage source, comprising measuring the outputvoltage of the RF power output unit; comparing the measured outputvoltage to at least one threshold voltage to produce a control signal;and adapting the output matching circuit by means of the control signalto operate the output unit below its saturation level.

It is possible to keep the RF power output unit, in particular thecollector of the RF output transistor, out of saturation by detectingthe output voltage, in particular the peak minimum collector voltage, ofthe RF power output unit.

In a preferred embodiment of the above method the adapting of the outputmatching circuit is done by changing either the magnitude or the phaseof the impedance transform function. The advantageous feature is thatthe magnitude and the phase are controlled in order to preserve thelinearity of the RF power output unit.

In a preferred embodiment of the above method the adapting of the outputmatching circuit and the adapting of the supply voltage are combinedwith a power amplifier efficiency optimization in case of a multiplethreshold detection by an analog-to-digital converter. The use of ananalog-to-digital converter which may be a part of the base-bandcontroller, allows a precise adapting of the output matching circuit andthe supply voltage.

In a preferred embodiment of the above method the output voltage of theRF power output unit is rectified before being compared to the thresholdvoltage in order to have a direct influence on the operating conditionof the power amplifier.

In a preferred embodiment of the above method the output voltage of theRF power output unit is compared to the threshold voltage by means of anoperational amplifier. The advantage is that the operational amplifiercompares the output voltage to the threshold voltage and amplifiesautomatically the difference between the two compared voltages.

In a preferred embodiment of the above method the output voltage of theRF power output unit is compared in at least two parallel operationalamplifiers to threshold voltages to produce at least two controlsignals, and wherein the at least two control signals are fed to thebase-band controller. The use of at least two parallel operationalamplifiers enables to determine more accurately the minimum collectorvoltage in order to preserve the linearity of the RF power output unit.

In a preferred embodiment of the above method the at least two thresholdvoltages have different voltage levels. The different voltage levels ofthe at least two threshold voltages enable to determine the minimumcollector voltage of the RF power output unit in order to preserve thelinearity of the RF power output unit.

In a preferred embodiment of the above method the supply voltage isadapted by a programmable DC-DC converter controlled by a base-bandcontroller which is fed by the control signal. A programmable DC-DCconverter controlled by a base-band controller is an advantageous wayfor a supply voltage adaptation.

The above object is achieved by a circuit for preserving linearity of aRF power amplifier wherein the power amplifier includes a RF poweroutput unit having a characteristic drive level, comprising a measuringunit measuring the output voltage of the RF power output unit; acomparing unit comparing the measured output voltage of the RF poweroutput unit to a threshold voltage to produce a control signal; a drivelevel adaptation unit adapting the drive level of the RF power outputunit or a supply voltage adaptation unit adapting a supply voltage ofthe RF power output unit to operate the output unit below its saturationlevel for preserving linearity of the RF power amplifier.

It is possible to keep the RF power output unit, in particular thecollector of the RF output transistor, out of saturation by detectingthe output voltage, in particular the peak minimum collector voltage, ofthe RF power output unit. This will preserve the linearity of the poweramplifier.

In a preferred embodiment of the above circuit the power amplifierincludes a variable gain preamplifier supplying the drive voltage to theRF power output unit; and wherein the control signal is fed from thecomparing unit to the preamplifier to adapt the gain of thepreamplifier. The variable gain preamplifier is used to keep the RFpower output unit out of saturation and thus preserving the linearity ofthe RF power output unit.

In a preferred embodiment of the above circuit a combining circuit isprovided between the comparing unit and the preamplifier combining thecontrol signal with the gain control signal of the preamplifier.

The above object is achieved by a circuit for stabilizing an antennacircuit comprising a RF power amplifier and a matching circuit, whereinthe RF power amplifier comprises a RF power output unit having acharacteristic drive level, comprising a measuring unit measuring theoutput voltage of the RF power output unit; a comparing unit comparingthe measured output voltage of the RF power output unit to a thresholdvoltage to produce a control signal; a drive level adaptation unitadapting the output matching circuit by means of the control signalthereby adapting the drive level of the RF power output unit to operatethe RF output unit below its saturation level for preserving linearityof the RF power amplifier.

It is possible to keep the RF power output unit, in particular thecollector of the RF output transistor, out of saturation by detectingthe output voltage, in particular the peak minimum collector voltage, ofthe RF power output unit and, if necessary, by adapting the poweramplifier output matching circuit. This will preserve the linearity ofthe power amplifier.

In a preferred embodiment of the above circuit the output matchingcircuit is configured to be adaptable with respect to either themagnitude or the phase of its impedance transform function. Theadvantageous feature of the output matching circuit is that themagnitude and the phase are controlled independently in order topreserve the linearity of the RF power output unit.

In a preferred embodiment of the above circuit a rectifier is providedbetween the RF power output unit and the comparing unit.

In a preferred embodiment of the above circuit the comparing unitcomprises an operational amplifier.

In a preferred embodiment of the above circuit at least two paralleloperational amplifiers are provided to produce at least two controlsub-signals, and wherein the at least two control sub-signals are fed tothe RF power output unit to adapt the gain thereof.

In a preferred embodiment of the above circuit the at least twothreshold voltages have different voltage levels.

In a preferred embodiment of the above circuit there are provided aDC-DC converter providing the supply voltage to the RF power outputunit; and a base-band controller between the comparing unit and theDC-DC converter which base-band controller is fed by the control signalwhereby the supply voltage is adapted.

In a preferred embodiment of the above circuit the output matchingcircuit comprises at least one load switching circuit.

In a preferred embodiment of the above circuit the load switchingcircuit comprises at least one PIN diode and at least one currentsource. A load switching circuit that consists of one or more PIN diodesis particularly suited for adaptation of the output matching circuit.

The advantages of the circuits according to the invention and of thepreferred embodiments thereof have been discussed above in connectionwith the methods according to the invention and of the preferredembodiments thereof.

The method of collector voltage detection can be used for linear poweramplifiers as used in CDMA, W-CDMA and Edge phones. The parameter toadapt (supply voltage, output match or power drive level) is, inprinciple, independent of the application. However, in GSM/Edge it iscommon to use a power control loop. Therefore, the proposed detectionmethod can more easily be combined with input drive level adaptationthan for the other systems.

These and various other advantages and features of novelty whichcharacterize the present invention are pointed out with particularity inthe claims annexed hereto and forming a part hereof. However, for abetter understanding of the invention, its advantages, and the objectobtained by its use, reference should be made to the drawings which forma further part hereof, and to the accompanying descriptive matter inwhich there are illustrated and described preferred embodiments of thepresent invention.

FIG. 1 shows a prior art block diagram of a power source isolated with acirculator from a mismatched antenna;

FIG. 2 shows a prior art power amplifier without a circulator;

FIG. 3 shows a block diagram of a power source of an antenna circuit fora basic discussion of collector voltage saturation;

FIG. 4 shows a block diagram of a power source of an antenna circuitaccording to a first embodiment of the invention;

FIG. 5 shows a block diagram of a power source of an antenna circuitaccording to a second embodiment of the invention;

FIG. 6 shows a block diagram of a power source of an antenna circuitaccording to a third embodiment of the invention; and

FIG. 7 shows a block diagram of a power source of an antenna circuitaccording to a forth embodiment of the invention.

FIG. 3 shows a block diagram of a power source of an antenna circuithaving a power amplifier and a matching circuit. The power amplifierthat is supposed to be just linear enough at maximum output power levelat a 50 Ohm antenna impedance, becomes non-linear under antenna mismatchconditions. As can be seen from FIG. 3, a main cause of distortion underantenna mismatch conditions is due to collector voltage saturation atthe final stage. Collector voltage saturation of the RF-transistoroccurs when the collector load impedance is relatively high incombination with high output power levels. The amount of saturationdepends on the magnitude and phase of the antenna reflection coefficientΓant and on losses in the matching network.

The invention is based on the findings that the RF-transistor collectorcan be kept out of saturation if the minimum peak voltage at thecollector is detect, the detected voltage is compared with a minimumallowable reference value and the gain of the power amplifier is reduced(adapted) to a level that just avoids “collector voltage underflow”conditions without re-introducing distortion, or increase the supplyvoltage of the power amplifier to a level that just avoids “collectorvoltage underflow” conditions by steering, preferably via the base-bandcontroller, a DC-DC converter that supplies the power amplifier, orchange the matching network between power amplifier and antenna under“collector voltage underflow” conditions to reduce the load impedance ofthe collector (load switch).

FIG. 4 shows a block diagram of a power source of an antenna circuitaccording to a first embodiment of the present invention. The embodimentcomprises a driver 2 connected with its output terminal to a baseterminal of a transistor 4. An emitter terminal of the transistor 4 isconnected to ground. A collector terminal of the transistor 4 isconnected to an inductance 6, a matching circuit 8 and a thresholddetection unit 16. The other side of the inductance 6 is connected to asupply voltage V_(sup). The matching circuit 8 is connected to anantenna 10 and with another terminal to ground. The threshold detectionunit is connected to an addition point 18. The result of the additionpoint 18 is fed to the driver 2. The threshold detection unit 16comprises a rectifier 12 connected on one side to the collector of thetransistor 4 and on the other side to a minus terminal of an operationalamplifier 14. A plus terminal of the operational amplifier 14 isconnected to a reference voltage V_(ref) which is equal to a minimumcollector voltage V_(col) _(—) _(min). The output of the operationalamplifier 14 is connected to the minus terminal of the addition point18. The rectifier 12 rectifies the voltage at the collector terminal ofthe transistor 4 and feeds the rectified voltage to the minus terminalof the operational amplifier 14. The operational amplifier forms thedifference between the rectified voltage of the rectifier 12 and thereference voltage at the plus terminal of operational amplifier 14. Theamplified output voltage of the operational amplifier 14 is fed to theaddition point 18. The addition point 18 forms the difference betweenthe output voltage of the operational amplifier 14, which is subtractedfrom the positive output signal of the gain control. The differentialsignal is fed to the driver 2. Depending on the differential signal ofaddition point 18 the driver controls the RF-voltage at the base (drivelevel) of the transistor 4. By detecting the collector voltage andreducing the gain of the transistor 4 the transistor 4 will be kept outof saturation.

FIG. 5 shows a block diagram of a power source of an antenna circuitaccording to a second embodiment of the invention. The embodimentcomprises a driver 22 controlled by a gain control 44, connected to abase terminal of a transistor 24. An emitter terminal of the transistor24 is connected to ground. A collector terminal of the transistor 24 isconnected to an inductance 26, a matching circuit 28 and a thresholddetection unit 36. The matching circuit 28 is connected to an antenna 30radiating the radiated power P_(rad). Another terminal of the matchingcircuit is connected to ground. The threshold detection unit comprises arectifier 32 connected on one side to the collector terminal of thetransistor 24 and on the other side connected to a minus terminal of anoperational amplifier 34. The rectifier 32 rectifies the collectorvoltage of transistor 24 and feeds the rectified voltage to the minusterminal of operational amplifier 34. A plus terminal of the transistor34 is connected to a reference voltage V_(ref) equal to a minimumcollector voltage V_(col) _(—) _(min). The operational amplifier 34amplifies the difference between the rectified collector voltage and thereference voltage. The amplified differential voltage is fed from theoperational amplifier 34 of the threshold detection unit 36 to a baseband controller 38. The base band controller 38 controls a DC/DCconverter 40. The DC/DC converter 40 is connected on one side to abattery voltage V_(bat) 42. Another terminal of the DC/DC converter 40is connected to the other side of the inductance 26. The DC/DC converter40 supplies the supply voltage V_(sup) via the inductance 26 to thecollector of the transistor 24.

In the embodiment of FIG. 5, the collector voltage of transistor 24 isdetected by the threshold detection unit 36, and below a given thresholdthe DC/DC converter 40 provides an output voltage which is increased tokeep transistor 24 out of saturation. The linearity preserving controlloop comprising the threshold detection unit 36, the base bandcontroller 38, the DC/DC converter 40 and the inductance 26 is also usedas an efficiency optimization loop. At low output power levels of thetransistor 24 the loop sets the DC/DC converter 40 to the correspondingminimum supply voltage and thus optimizes efficiency.

FIG. 6 shows a block diagram of a power source of an antenna circuitaccording to a third embodiment of the invention. The embodiment of FIG.6 is rather similar to the embodiment of FIG. 5. Therefore equal partshave equal numbers. The differences between FIG. 6 and FIG. 5 are adifferent threshold detection unit 52 and a different base bandcontroller 54. The threshold detection unit 52 comprises a rectifier 46connected on one side to the collector of the transistor 24 and on theother side to the minus terminal of an operational amplifier 48 and to aminus terminal of an operational amplifier 50. The rectifier 46rectifies the collector voltage of transistor 24 like the rectifier 32in FIG. 5. The rectified voltage is fed to two operational amplifiers 48and 50. The two operational amplifiers 48 and 50 have differentreference voltages connected to their plus terminals. The operationalamplifier 48 is connected to a reference voltage V_(ref1) equal to 0.5 Vand the plus terminal of the operational amplifier 50 is connected to areference voltage V_(ref2) equal to 1 V. The amplified differentialvoltage between the rectified voltage of rectifier 46 and the referencevoltage is provided to the base band controller 54. The amplifieddifferential signal X of amplifier 48 and the amplified differentialvoltage Y of amplifier 50 is fed to the base band controller 54. Thebase band controller 54 has two input terminals for the two differentamplified differential voltages X and Y. The base band controller 54controls the DC/DC converter 40 depending on the voltages X and Y.

In the embodiment shown in FIG. 6, several threshold levels instead ofone threshold level in FIG. 5, are used to determine more accurately theminimum collector voltage in order to adapt the supply voltage. FIG. 6shows a minimum configuration that allows the optimization of efficiencyas well as linearity. Table 3 shows the three levels that can bedistinguished.

TABLE 3 X Y Condition Remark 0 0 V_(col) _(—) _(min) > 1 V Too high forgood efficiency 0 1 0.5 V < V_(col) _(—) _(min) < 1 V Optimum 1 1V_(col) _(—) _(min) < 0.5 V Too low for good linearity

The usage of an analogue detector output and an analogue to digitalconverter allows a more analogue control of the supply voltage. Thesettling time of the DC/DC converter should be sufficiently short.

The base band controller 54 can use “old” threshold detectioninformation and the “used” information on required output power todetermine an optimum output voltage value for the DC/DC converter whichmight need adaptation when “new” threshold detection information iscollected.

FIG. 7 shows a block diagram of a power source of an antenna circuitaccording to a forth embodiment of the invention. The embodimentcomprises a driver 56 controlled by a gain control 58 connected to abias terminal of a transistor 62. An emitter terminal of the transistor62 is connected to ground. A collector terminal of transistor 62 isconnected to an inductance 60, a matching circuit 70 and a rectifier 64of a threshold detection unit 68. The other terminal of the inductance60 is connected to a supply voltage V_(sup). The rectifier 64 rectifiesthe voltage at the collector terminal of transistor 62. The rectifiedvoltage is fed to a minus terminal of an operational amplifier 66. Areference voltage V_(ref) equal to a minimum collector voltage V_(col)_(—) _(min) is connected to a plus terminal of the operational amplifier66. The operational amplifier amplifies the difference between therectified voltage and the reference voltage V_(ref). The amplifieddifferential voltage is fed to the matching circuit 70. Another terminalof the matching circuit 70 is connected to an antenna 72 radiating thepower P_(rad).

The threshold detection unit detects the collector voltage of thetransistor 62 and the matching circuit 70 is changed below a giventhreshold to keep the transistor 62 out of saturation. In the matchingcircuit 70 either the magnitude or the phase of impedance informationbetween the output impedance of the transistor 62 and the inputimpedance of the antenna can be changed and vice versa.

A relatively simple adaptation mechanism changing the magnitude ratherthan the phase can be realized by means of a load switch changing thecollector impedance from high to low when voltage underflow occurs. Thisload switch can be built up with a Pin diode and a current source. Asmoother control can be obtained by using multiple threshold detectionand by more than one switch. An advantage of adaptation of the outputmatching is that a relatively large radiated output power can beobtained under antenna mismatch conditions while maintaining linearity.

An implementation with multiple threshold detection levels and multipleswitches can also be used to optimize the efficiency. The load lineshall be increased to a minimum allowable collector peak voltage.

New characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many respects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size, and arrangement of parts, without exceeding the scope ofthe invention. The scope of the invention is defined in the language inwhich the appended claims are expressed. The word “comprising” does notexclude the presence of other elements or steps than those listed in aclaim, and “a” or “an” does not exclude a plurality.

1. A method for preserving linearity of a RF power amplifier, the poweramplifier including a RF power output unit having a characteristic drivelevel and fed by a supply voltage, comprising: measuring the outputvoltage of the RF power output unit; comparing the measured outputvoltage to at least one threshold voltage to produce a control signal;and selecting between reducing the drive level of the RF power outputunit responsive to the control signal and increasing the supply voltageof the RF power output unit responsive to the control signal to therebyoperate the output unit below its saturation level.
 2. The method ofclaim 1, wherein the power amplifier includes a variable gainpreamplifier supplying the drive voltage to the RF power output unit andwherein the control signal is used to adapt the gain of thepreamplifier.
 3. The method of claim 2, wherein the control signal iscombined with the gain control signal of the preamplifier.
 4. The methodof claim 1, wherein the output voltage of the RF power output unit isrectified before being compared to the threshold voltage.
 5. The methodof claim 1, wherein the output voltage of the RF power output unit iscompared to the threshold voltage by means of an operational amplifier.6. The method of claim 5, wherein the output voltage of the RF poweroutput unit is compared in at least two parallel operational amplifiersto threshold voltages to produce at least two control signals, andwherein the at least two control signals are fed to a base-bandcontroller.
 7. The method of claim 6, wherein the at least two thresholdvoltages have different voltage levels.
 8. The method of claim 1,wherein the supply voltage is adapted by a programmable DC-DC convertercontrolled by a base-band controller which is fed by the control signal.9. A method for controlling an antenna circuit comprising a RF poweramplifier and a matching circuit by preserving linearity of a RF poweramplifier, the power amplifier comprising a RF power output unit havinga characteristic drive level and fed by a supply voltage source,comprising: measuring the output voltage of the RF power output unit;comparing the measured output voltage to at least one threshold voltageto produce a control signal; and adapting the output matching circuit byselecting between reducing the drive level of the RF power output unitresponsive to the control signal and increasing the supply voltage ofthe RF power output unit responsive to the control signal, therebyoperating the output unit below its saturation level.
 10. The method ofclaim 9, wherein adapting of the output matching circuit includeschanging a magnitude or a phase of an impedance transform function. 11.The method of claim 9, wherein adapting the output matching circuitincludes optimizing a power amplifier efficiency responsive to multiplethreshold detection by an analog-to-digital converter.
 12. A circuit forpreserving linearity of a RF power amplifier wherein the power amplifierincludes a RF power output unit having a characteristic drive level,comprising: a measuring unit measuring the output voltage of the RFpower output unit; a comparing unit comparing the measured outputvoltage of the RF power output unit to a threshold voltage to produce acontrol signal; an adaptation unit configured to operate the output unitbelow its saturation level for preserving linearity of the RF poweramplifier, the adaptation unit including a drive level adaptation unitfor reducing the drive level of the RF power output unit and a supplyvoltage adaptation unit for increasing a supply voltage of the RF poweroutput unit.
 13. The circuit of claim 12, wherein the power amplifierincludes a variable gain preamplifier supplying the drive voltage to theRF power output unit, and wherein the control signal is fed from thecomparing unit to the preamplifier to adapt the gain of thepreamplifier.
 14. The circuit of claim 13, comprising a combiningcircuit between the comparing unit and the preamplifier to combine thecontrol signal with the gain control signal of the preamplifier.
 15. Acircuit for stabilizing an antenna circuit comprising a RF poweramplifier and a matching circuit, wherein the RF power amplifiercomprises a RF power output unit having a characteristic drive level,comprising: a measuring unit measuring the output voltage of the RFpower output unit; a comparing unit comparing the measured outputvoltage of the RF power output unit to a threshold voltage to produce acontrol signal; and an adaptation unit configured to adapt the outputmatching circuit responsive to the control signal to thereby operate theRF output unit below its saturation level for preserving linearity ofthe RF power amplifier, the adaptation unit including a drive leveladaptation unit and a supply voltage adaptation unit.
 16. The circuit ofclaim 15, wherein the output matching circuit is configured to beadaptable with respect to either a magnitude or a phase of its impedancetransform function.
 17. The circuit of claim 12, comprising a rectifierbetween the RF power output unit and the comparing unit.
 18. The circuitof claim 12, wherein the comparing unit comprises an operationalamplifier.
 19. The circuit of claim 18, comprising at least two paralleloperational amplifiers to produce at least two control sub-signals, andwherein the at least two control sub-signals are fed to a base-bandcontroller to adapt the gain of the RF power output unit to adapt thegain thereof.